The present invention relates to a moistureproof multilayer film having high moistureproofness, and more particularly to a moistureproof multilayer film suitable for use as a packaging material and a sealing material for food, medicine, electronic components and so on, and to a method for producing the same. The moistureproof multilayer film of this invention can have moistureproofness comparable to a poly(chlorotrifluoroethylene) film and is particularly suitable for use as a packaging film for sealing electronic components such as an electroluminescence element.
Conventionally, a composite film including a synthetic resin film and a vapor deposited film of an inorganic oxide or a metal formed on a surface of the synthetic resin film is used as a packaging material and a sealing material for food, medicine, electronic components and so on because it has good oxygen gas and water vapor barrier properties, excellent transparency, and, when the vapor deposited film is a silicon oxide thin film or the like, suitability for use in microwave ovens. When synthetic resin films of various types are laminated on one or both sides of a composite film including a vapor deposited film on at least one side thereof, the vapor deposited film can be protected, and various functions such as strength, thermal resistance, heat seal properties can be imparted to the resulting multilayer film.
However, conventional composite films including a vapor deposited film cannot have sufficiently low moisture permeability and thus are not necessarily suitable for use as a packaging film for sealing electronic components which are apt to require high moistureproofness. This problem will be described in detail taking an electroluminescence element (EL element) as an example.
A phenomenon in which a solid luminescent compound (phosphor) converts electric energy into luminescent energy when an electric field is applied thereto is referred to as electroluminescence. Electroluminescence elements can be classified into a thin film type and a dispersion type according to their basic element structure. A thin film type EL element has a luminescent layer of a thin film of a phosphor. A dispersion type EL element has a luminescent layer of a powdery phosphor dispersed in an organic or inorganic binder. An EL element has an element body comprising a luminescent layer sandwiched directly or through insulating layers between a pair of electrodes, and a transparent electrode is used as at least one of the paired electrodes. When the phosphor constituting the luminescent layer absorbs moisture, the luminance (brightness) thereof is considerably lowered. Thus, an EL element generally has such a structure that an EL element body comprising a luminescent layer sandwiched between a pair of electrodes is covered (sealed) with a transparent moistureproof material.
As the moistureproof material for EL elements, poly(chlorotrifluoroethylene) (PCTFE) film or glass substrate has conventionally been used. PCTFE film has the highest moistureproofness in synthetic resin films but is high in cost, and its moistureproofness is considerably decreased when atmospheric temperature exceeds 50xc2x0 C. Thus, the service life of an EL element sealed with PCTFE film is extremely shortened under high temperature and high moisture conditions. As for glass substrate, there is a limit to reducing the thickness and weight, and it is lacking in flexibility. Also, it is difficult to seal an EL element body only with glass substrate, and it must be used in combination with a moistureproof film having flexibility.
Thus, there has been a demand for development of a moistureproof film having high moistureproofness substitutable for PCTFE film. Some materials, including polyvinylidene chloride and polyvinyl alcohol, have been studied for a resin material substitutable for PCTFE. Synthetic resin films on which a vapor deposited film of an inorganic oxide or a metal is provided have also been studied. However, it has been difficult to develop a moistureproof material having moderate flexibility and high moistureproofness comparable to PCTFE film.
For example, JP-A-H08-300549 discloses a method of improving gas barrier properties of a laminated film comprising at least biaxially stretched plastic film and skin film/adhesive resin layer/flexible film, in which the film having gas barrier properties and provided on the biaxially stretched plastic film is in contact with the adhesive resin layer. The method comprises heating the biaxially stretched plastic film at a temperature between its glass transition point and melting point so that the biaxially stretched plastic film exhibits an elongation and shrinkage in the longitudinal direction (MD) of within 2% and within 5%, respectively. In the above publication, vapor deposited films of inorganic oxides or metals are shown as examples of films having gas barrier properties.
In the method disclosed in the above publication, the elongation or shrinkage of the laminated film is controlled by passing the laminated film, during the heating of the laminated film, between two independently driven rolls A and B while adjusting the rotational speeds of the rolls. More specifically, the circumferential velocity of the roll B is set at 1.02 to 0.95 times that of the roll A so that the laminated film may be heated under tension. In the above method, the heating temperature is preferably the glass transition point plus 70xc2x0 C. or higher and not higher than the melting point of the biaxially stretched plastic film. In the above disclosure, an example in which a laminated film was brought into contact with heat rolls having a temperature of 230xc2x0 C. for 1.5 seconds (Example 1), an example in which a laminated film was brought into contact with heat rolls having a temperature of 190xc2x0 C. for 3.2 seconds (Example 2), and an example in which a laminated film was brought into contact with heat rolls having a temperature of 250xc2x0 C. for 0.9 seconds (Example 3) are shown. In other examples, laminated films were heated in a heating furnace at 170xc2x0 C. for 10 minutes (Example 4) and at 210xc2x0 C. for 2 minutes (Example 5), respectively, while being passed between two independently driven rolls slowly. By the heat treatment, the moisture permeability (as measured by a cup method, under conditions of a temperature of 40xc2x0 C. and a relative humidity of 90%) of the laminated film was improved from 1.5 g/m2.day to 0.6 g/m2.day (Example 4). However, the moisture permeability is far inferior to that of PCTFE film (0.03 g/m2.day).
Recently, the present inventors found that a moistureproof film having high moistureproofness can be obtained by drying a transparent multilayer film including a layer of a moisture absorbing resin such as polyvinyl alcohol and thin films of an inorganic oxide provided directly or through an adhesive layer on both sides of the moisture absorbing resin layer (PCT/JP98/01781). However, the moistureproof film requires the use of a moisture absorbing resin, so that there is a limitation on selection of resin materials. Also, in order to reduce the amount of moisture which can permeate the moistureproof film as well as to lower its moisture permeability, the moisture absorbing resin layer must be dried to a completely dried state by high-level drying treatments including vacuum drying.
It is, therefore, an object of the present invention to provide a moistureproof multilayer film having high moistureproofness, essentially comprising a composite film including a non-moisture absorbing resin layer and vapor deposited film of an inorganic oxide or a metal formed at least one side of the non-moisture absorbing resin layer. Another object of this invention is to provide a method for producing the moistureproof multilayer film.
As a result of zealous studies to solve the problems of the prior arts, the present inventors found that a moistureproof multilayer film having high moistureproofness comparable to that of PCTFE film and moderate flexibility and softness can be obtained by a method comprising preparing a multilayer film comprising a composite film which includes a non-moisture absorbing resin film and a vapor deposited film of an inorganic oxide or a metal formed on at least one side of the non-moisture absorbing resin film and which has such a layer structure that the composite film is laminated through an adhesive layer on another non-moisture absorbing resin layer or on another composite film with the vapor deposited films thereof facing each other, in which a total number (n) of the deposited films laminated adjacent to the adhesive layer is 2 to 8, and heat-treating the multilayer film at a relatively low temperature of lower than 140xc2x0 C. for at a long time.
According to the method of this invention, the heat treatment of the multilayer film does not have to be performed under tension but can be carried out by allowing it to stand in a heating furnace in a rolled-up state, for example. Thus, the method of this invention is suitable for mass-production. Also, in the method of this invention, the heat treatment can be carried out at a relatively low temperature 55xc2x0 C. or higher and lower than 140xc2x0 C. According to the results of experiments conducted by the present inventors, when the multilayer film is subjected to a high-temperature heat treatment, the moisture permeability thereof tends to lower. Especially, when the multilayer film is heat-treated at as high a temperature as 170xc2x0 C. or higher, the moisture permeability thereof considerably lowers. Additionally, in the method of this invention, a moisture absorbing resin layer is not essential, and there is no need to dry a moisture absorbing resin layer until it is completely dried. The moistureproof multilayer film of this invention can have flexibility suitable for use as, for example, a moistureproof material for an EL element body by adjusting the total number of vapor deposited films within a moderate range. This invention has been made based on these findings.
According to the present invention, there is provided a moistureproof multilayer film, comprising a composite film having a non-moisture absorbing resin layer and a vapor deposited film of an inorganic oxide or a metal formed on
at least one side of the non-moisture absorbing resin layer, the multilayer film having such a layer structure where the vapor deposited film surface of the composite film is laminated through an adhesive layer on a vapor deposited film surface of another composite film or a surface of another non-moisture absorbing resin layer,
in which a total number (n) of the deposited films laminated adjacent to the adhesive layer is 2 to 8, and
in which a moisture permeability (W; unit: g/m2.day) as measured under conditions of a temperature of 40xc2x0 C. and a relative humidity of 100% satisfies the relation represented by the equation (1):
Wxe2x89xa6(1/n)xc3x970.20.xe2x80x83xe2x80x83(1)
According to the present invention, there is also provided a method for producing a moistureproof multilayer film comprising a composite film having a non-moisture absorbing resin film and a vapor deposited film of an inorganic oxide or a metal formed on at least one side of the non-moisture absorbing resin layer, the method comprising:
(1) preparing a multilayer film having such a layer structure where the vapor deposited film surface of the composite film is laminated through an adhesive layer on a vapor deposited film surface of another composite film or a surface of another non-moisture absorbing resin layer, wherein a total number (n) of the vapor deposited films laminated adjacent to the adhesive layer is 2 to 8, and
(2) subjecting the multilayer film to a heat treatment in a hot dry atmosphere at a temperature lower than 140xc2x0 C. but not lower than 55xc2x0 C. for at least 10 hours.
The present invention will fully be understood, referring to the following detailed description. Further extensive applications of the invention will be apparent from the following description in details. However, it should be noted that the detailed description and specific examples are preferred embodiments of the invention, only for the purpose of the description thereof. Because it is apparent for the person ordinary skilled in the art to modify and change in variety of manners, within the scope and spirits of the invention. The applicant does not intend to dedicate any disclosed embodiments to the public, and to the extent any disclosed modifications or alternations may not literally fall within the scope of the Claims, they are considered to be part of the invention under the doctrine of the equivalents.